1. Field of the Invention
The present invention relates to a resin gear manufactured by injection molding in which a molten material is injected into a cavity of a mold through a plurality of gates.
2. Description of Related Art
Heretofore, this type of technique may include the techniques disclosed in Japanese Patents No. 3364873 and 3387218 and Japanese unexamined Utility Model application publication No. 4(1992)-80956. Of them, for example, Japanese Patent No. 3364873 discloses one example of electronically controlled throttle valve systems. This system comprises a throttle valve, an actuator for actuating the throttle valve, and a throttle body which houses and supports the throttle valve and the actuator. The throttle valve is fixed to a throttle shaft rotatably supported in the throttle body. An output shaft of the actuator is drivingly coupled to the throttle shaft via a gear transmission mechanism including a plurality of gears. This system is arranged to rotate the output shaft of the actuator, rotating the throttle shaft through the gear transmission mechanism, thereby opening/closing the throttle valve. Herein, in order to accurately open/close the throttle valve in association with the rotation of the output shaft of the actuator, each gear of the gear transmission mechanism has to be machined with high accuracy. In recent years, a resin gear is often used because of a demand for weight saving. A resin gear made of resin by injection molding particularly has an issue of molding accuracy such as roundness.
According to Japanese Patent No. 3387218, when a resin molded component such as a gear is to be manufactured by injection molding, a molten material is injected into a cavity of a mold through a plurality of gates arranged at almost equally spaced intervals circumferentially on a piece to be molded. The resin material has a thermal shrinkage rate different between a direction of flow and a direction perpendicular to the flow. In the above case, accordingly, it would be considered to be extremely difficult to achieve dimensional accuracy of the molded piece, in particular the accuracy of a peripheral part thereof.
According to Japanese Patent No. 3387218, further, the roundness of the molded piece also largely depends on the resin material. Specifically, to enhance rigidity and elasticity of gears, fibrous additives such as glass and carbon fiber are often mixed to resin. The molten material injected into the cavity usually spreads from each gate, and thus merging lines (weld lines) are formed extending in a radial direction between the gates, as many as the number of gates. Accordingly, the additives are oriented along the weld lines in the weld line region and oriented in a direction substantially perpendicular to the weld lines in other areas. This orientation induces a difference in shrinkage of the resin in each angular position, which makes it difficult to provide high roundness of the peripheral part. This problem of roundness is apparent in a rim in the peripheral part of the gear and more or less found in a boss in the central part of the gear. It can be considered that the reason why the difference in distribution of the shrinkage rate causes difficulty in providing high roundness is how the molten material flows in the cavity. The molten resin is normally supposed to flow radially outwardly in every directions from the center; however, this flow pattern is substantially impossible for the molded piece such as a gear formed with a boss. Thus, the plurality of gates is usually arranged on a disk face of the gear to be molded. If those gates are positioned closely around the boss, the time needed for injection of the molten resin into the cavity will be longer and it is also hard to control the temperature during the injection. Further, this configuration has a narrow space between the adjacent gates, resulting in a difficulty in machining a mold itself. It is accordingly conceivable to arrange many gates circumferentially almost midway between the rim and the boss of the gear. This configuration could also has the following problems if many gates are closely spaced that: (1) the temperature on the side of the piece being molded where the gates are arranged for injection of the material extremely rises (the temperature of the molten material is about 320° C.), which causes a large difference in temperature from the opposite side, leading to a difference in shrinkage during cooling; (2) many runners are needed, deteriorating utilization efficiency of the material; and (3) the number of machining steps for manufacture of the mold is increased.
Japanese Patent No. 3387218 therefore discloses a resin gear having excellent roundness, which can be manufactured in a manner that allows the molten material to uniformly flow radially inwardly and outwardly from positions circumferentially arranged between the rim and the boss without requiring an increase in number of gates. To be concrete, as shown in FIG. 16, this gear is formed with a boss 62 at the center of a disk 61 and a rim 63 in the outer peripheral part. Between the boss 62 and the rim 63, a circular rib 64 is formed concentrically with them. Further, thick-wall portions 65 and 66 are concentrically formed between the rim 63 and the circular rib 64 and between the circular rib 64 and the boss 62 respectively. The intermediate portions between the inner thick-wall portion 65 and the circular rib 64 and between the outer thick-wall portion 66 and the circular rib 64 form thin recessed portions 67 and 68 respectively. The molten resin is injected into the cavity in a direction perpendicular to the side of the disc 61 through a plurality of gates (gate marks 69) are arranged in circumferentially angular positions at almost equally spaced intervals on the circular rib 64. Radial ribs 70 are formed almost midway between the positions of injection through the gates (the gate marks 69) and extending radially between the thick-wall portions 65 and 66 via the circular rib 64.
In the aforementioned conventional gear, the radial ribs 70 formed between the gates (the gate marks 69) are of large thickness and large area. This would cause the pressure of the molten material to disperse, resulting in a decreased molding-injection force, so that the radial flow of the molten material is induced through the radial ribs 70 with the result that longer weld lines are formed. The length of each weld line exerts a large influence on the shrinkage property (anisotropy) of the molten resin. Thus, there is a possibility that the strength and the dimensional accuracy of the gear are deteriorated. The recessed portions 67 and 68 formed inside and outside of the circular rib 64 are uniform in width and small in thickness. During molding, therefore, the flow of the molten material is likely to become slow as coming away from the gates and be injected nonuniformly, which causes a deterioration of dimensional accuracy of the gear. Such deterioration of gear strength and dimensional accuracy would affect on opening/closing accuracy and smooth operations of the throttle valve in the electronically controlled throttle valve system.